1. Field of the Invention
This invention relates to protection of integrated circuits and, in particular, to the corrosion and electrostatic discharge protection of integrated circuits.
2. Art Background
Integrated circuits and their assemblages are typically handled, shipped, and stored in packaging material such as rigid containers, plastic bubble holders sealed with a plastic tape (denominated "tape and reel carriers") plastic bags, and polymer foam. For a wide variety of integrated circuits, electrostatic charge/discharge and possibly corrosion protection must be provided to avoid destruction or serious degradation of the integrated circuit during storage, shipping, or use. For example, static electricity discharge from a person to a device being handled (an occurrence common during the winter season) is often sufficient to produce such damage. Additionally, for devices having exposed metallization that readily corrodes, gases in the air including hydrogen chloride, chlorine and hydrogen sulfide cause degradation, especially when in the presence of water vapor.
Various means have been attempted to provide electrostatic or corrosion protection. In the case of corrosion protection, generally a material containing a volatile organic material is placed in the same shipping container as the integrated circuit. These volatile organic materials such as fatty acids coat the leads of the integrated circuits. The effects of these organic materials on the leads is variable, i.e., at times the organic materials corrode the leads and at other times the organic materials provide some corrosion protection. However, the organic material, because of its volatility, is transient; hence, meaningful protection over a substantial period of time is often lacking. Additionally, the organic material contaminates exposed metal and thus hinders subsequent soldering.
A typical approach for providing electrostatic protection involves the surface metallization of a plastic packing material such as a polyethylene bag with, for example, aluminum. Although this approach yields some electrostatic protection, it is expensive and typically protection is limited because static charge is dissipated too rapidly and the potential for arcing to the device is enhanced. Volatile organic coatings are also employed for electrostatic protection but induce time and humidity dependent variations in surface resistance of the device and have the same shortcomings as result from their use for corrosion protection. Use of an organic polymer in configurations such as polymer bags impregnated with carbon is yet another approach to discharge protection. However, the conductivity of such materials is disadvantageously high for static dissipation purposes, e.g., less than 10.sup.4 ohms/square, and this conductivity is such a highly nonlinear function of carbon black concentration that a desired conductivity is difficult to achieve reproducibly by conventional manufacturing techniques. Further, the carbon black sheds from the polymer and makes such materials unacceptable for most clean room environments. Thus, the protection of many integrated circuits is less than entirely desirable.
A polymer guard for electrostatic sensitive portions of an integrated circuit device is disclosed in U. S. Pat. No. 5,154,886 to Franey et al. (Franey et al.). The protective coating described in Franey et al. is a polymer matrix impregnated with carbon black particles and a metal that undergoes chemical bonding with the carbon. Although the material described in Franey et al. provides adequate protection, the polymer film is formed into a configuration such as a bag, a rigid container, or a tape. In this example, the minimum thickness of the polymer product is typically 2 mils (0.002 inches), and can be as thick as 500 mils (0.5 inches). In certain instances, it is preferable if the coating is applied directly on the article, rather than placing the article in a container made of the polymer guard. Therefore, a polymer guard that can be applied directly onto the article to be protected is desired.